Generally, storage subsystems are equipped with randomly accessible nonvolatile storage media. Examples of such randomly accessible nonvolatile storage media include magnetic disk drives and optical disk drives. The current main stream of storage subsystems is equipped with multiple hard disk drives (HDDs).
Along with the advancement of semiconductor technology, nonvolatile semiconductor memories that can be used as storage media in storage subsystems are being developed. One example of drives using such nonvolatile semiconductor memories as storage media is a solid state drive (SSD). The storage subsystem equipped with SSDs have superior power saving property and access time compared to storage subsystems only equipped with hard disk drives (HDDs).
Storage subsystems have controllers for controlling data transfer between a host computer connected to the storage subsystem and the storage media disposed within the storage subsystem. The controller is equipped with a frontend interface for connecting the host computer, a backend interface for connecting a large number of drives as storage media, a processor for controlling the storage subsystem, a memory connected to the processor, and so on. A SAS (Serial Attached SCSI) is known as a standard of the communication network for connecting backend interfaces and the like.
A SAS backend network includes an initiator, drives such as SSDs and HDDs, and an expander (hereinafter also abbreviated as “Exp”). The initiator is a device for outputting command requests such as read/write to the drive, and it can be included in the backend interface of the controller, for example. The expander is a switch for connecting multiple drives to the initiator. In order to connect a large number of drives, multiple expanders are mutually connected and used.
In the prior art storage subsystem, expanders are generally connected in a cascade (in series) in order to connect a large number of drives. Patent Literature 1 discloses a method for connecting expanders in a cascade.